1. Field of the Invention
The present invention relates to electro-luminescence display devices and methods of driving the same, and more particularly to an electro-luminescence display device and a method of driving the same, capable of compensating differences in brightness to which pictures are displayed by panels in an electro-luminescence display device.
2. Description of the Related Art
Until recently, cathode ray tubes (CRTs) have generally been used in display systems. However, use of newly developed flat panel displays such as liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and electro-luminescence (EL) devices are becoming increasingly common due to their low weight, thin dimensions, and low power consumption.
Generally, EL devices are self-luminous devices and include fluorescent bodies capable of emitting light when electrons are recombined with holes. Depending on the compounds used in the fluorescent body, EL devices are classifiable as inorganic EL devices, containing inorganic compounds, or as organic EL devices, containing organic compounds. Generally, EL devices have excellent response speeds and light emission characteristics and are capable of displaying images at a high brightness and over wide ranges of viewing angles. Therefore, it can be reasonably anticipated that EL devices will be widely used in the future.
Organic EL devices typically include an electron injection layer, an electron transport layer, a light-emitting layer, a hole transport layer, and a hole injection layer arranged between a cathode and an anode. If a specific voltage is applied between the anode and the cathode of an organic EL device (OELD), electrons generated at the cathode migrate to the light-emitting layer via the electron injection and electron transport layers while holes generated at the anode migrate to the light-emitting layer via the hole injection and hole transport layers. Accordingly, electrons and holes supplied from the electron and hole transport layers are recombined in the light-emitting layer, causing the light-emitting layer to emit light.
FIG. 1 illustrates a schematic view of an active matrix related art organic electro-luminescence device.
Referring to FIG. 1, a related art organic EL device (OELD) can be provided as an active matrix type display and include an EL panel 20 having a plurality of pixels 28 arranged at areas defined by crossings of scan lines SL and data lines DL, a scan driver 22 for driving the scan lines SL of the EL panel 20, a data driver 24 for driving the data lines DL of the EL panel 20, and a gamma voltage generator 26 for applying a plurality of gamma voltages.
Generally, the scan driver 22 sequentially applies scan pulses to the scan lines SL and the data driver 24 converts externally inputted digital data signals into analog data signals using the gamma voltages applied from the gamma voltage generator 26. Further, the data driver 24 applies the analog data signals to the data lines DL in synchrony with the application of the scan pulses. Each of the plurality of pixels 28 receives an analog data signal from the data lines DL and, when the scan pulses are applied to the scan lines SL, generate light corresponding to the received analog data signal.
FIG. 2 illustrates a related art pixel within the active matrix related art organic electro-luminescence device shown in FIG. 1.
Referring to FIG. 2, each pixel 28 includes an organic electro-luminescence (OEL) cell having a cathode connected to a ground voltage source GND, a cell driver 30 connected to a scan line SL, data line DL, a supply voltage source VDD, and an anode of the OEL cell for driving the OEL cell.
The cell driver 30 includes a switch thin film transistor (TFT) T1 having a gate terminal connected to the scan line SL, a source terminal connected to the data line DL, and a drain terminal connected to a first node N1; a drive TFT T2 having a gate terminal connected to the first node N1, a source terminal to the supply voltage source VDD, and a drain terminal connected to the OEL cell; and a capacitor C connected between the supply voltage source VDD and the first node N1.
When a scan pulse is applied from the scan line SL, the switch TFT T1 is turned on and an analog data signal applied from the data line DL is transmitted to the first node N1. The analog data signal applied to the first node N1 is then simultaneously charged to the capacitor C and applied to the gate terminal of the drive TFT T2. In response to the analog data signal applied from the data line DL, the drive TFT T2 controls the amount of current, I, that is applied to the OEL cell from the supply voltage source VDD. By controlling the amount of current applied to the OEL cell, the drive TFT T2 controls the luminescence characteristics of the OEL cell. When the switch TFT T1 is turned off, the analog data signal stored by the capacitor C is discharged, enabling the drive TFT T2 to apply the current, I, from the supply voltage source VDD to the OEL cell. Accordingly, the luminescence characteristics of the OEL cell are maintained uniformly until an analog data signal of a successive frame is applied from the data line DL.
As described above, the related art electro-luminescence applies current signals to each of the OEL cells, wherein the strength of the applied current corresponds to digital data signals inputted into the data driver. Upon applying the current to the OEL cells, the electro-luminescence displays pictures. The related art OELD displays color by providing the OEL cells as R OEL cells having a red (R) fluorescent body, G OEL cells having a green (G) fluorescent body, and B OEL cells having a blue (B) fluorescent body, wherein sets of individual R, G, and B OEL cells are combined within a pixel.
The efficiency with which each of the R, G, and B fluorescent bodies emit light vary with the color of each fluorescent body. Accordingly, when analog data signals having a constant level are applied to the R, G, and B OEL cells, the R OEL cells emit light at a different brightness than the G OEL cells, the G OEL cells emit light at a different brightness than the B OEL cells, and the B OEL cells emit light at a different brightness than the R OEL cells. Therefore, gamma voltages are generally applied by the gamma voltage generator 26 to equalize the brightness at which each set of R, G, and B OEL cells emit light, enabling a pixel containing R, G, and B OEL cells to express white light. Related art gamma voltage generators 26 generally include gamma voltage suppliers that generate gamma voltages specific to each R, G, and B OEL cell within the pixel.
FIG. 3 illustrates a detailed circuit configuration of a first type within the related art gamma voltage generator shown in FIG. 1.
Referring to FIG. 3, the related art gamma voltage generator 26 shown in FIG. 1 includes a plurality of gamma voltage suppliers (e.g., R, G, and B, gamma voltage suppliers) corresponding to each of the R, G, and B OEL cells. Each of the plurality of gamma voltage suppliers generates n number of gamma voltages GAMMA1 to GAMMAn (where n is a natural number). The n gamma voltages are then used to generate analog data signals having different brightness levels, in correspondence with the digital data signals externally inputted to the data driver. For convenience of illustration, however, only one gamma voltage supplier is illustrated. Within the related art gamma voltage generator, each gamma voltage supplier includes a plurality of resistor pairs R1R2, R3R4, R5R6, R7R8, . . . , R2n–1R2n connected to one another in parallel between the supply voltage source VDD and the ground voltage source GND. The plurality of resistor pairs divide a supply voltage applied from the supply voltage source VDD and generate the n gamma voltages GAMMA1 to GAMMAn. Subsequently, the n gamma voltages are applied to the data driver 24. Electromagnetic noise of the gamma voltages GAMMA1 to GAMMAn, generated by the resistor pairs R1R2, R3R4, R5R6, R7R8, . . . , R2n–1R2n can be eliminated by the amplifiers 31 to 35 before the gamma voltages GAMMA1 to GAMMAn are applied to the data driver 24. The data driver 24 converts externally inputted digital data signals into analog data signals using any one of the gamma voltages GAMMA1 to GAMMAn. Subsequently, the converted analog data signals are applied to the data lines DL, causing predetermined pictures to be displayed by the EL panel 20.
FIG. 4 illustrates a detailed circuit configuration of a second type within the related art gamma voltage generator shown in FIG. 1.
Referring to FIG. 4, the gamma voltage generator 26 includes a single gamma voltage supplier for generating n number of gamma voltages GAMMA1 to GAMMAn. The n gamma voltages are then used to generate analog data signals having different brightness levels, in correspondence with the digital data signals externally inputted to the data driver. Accordingly, the gamma voltage supplier includes (n+1) number of resistors R11, R12, R13, R14, . . . , R1n+1 connected in series between the supply voltage source VDD and a ground voltage source GND to generate n number of gamma voltages GAMMA1 to GAMMAn. Subsequently, the n gamma voltages GAMMA1 to GAMMAn are applied to the data driver 24. The data driver 24 generates analog data signals using gamma voltages GAMMA1 to GAMMAn in correspondence with the externally inputted digital data signals. Application of the generated analog data signals to the data lines DL is synchronized with the application of the scan signals, causing predetermined pictures to be displayed by the EL panel 20.
Within the related art electro-luminescence device described above, the amount of current, I, flowing to each of the OEL cells is determined by the gate voltage of the drive TFT T2 (i.e., the voltage of the analog data signal applied to the gate terminal of the drive TFT T2). However, the amount of current, I, that is transmitted by the drive TFT T2 is influenced by a threshold voltage Vth of the drive TFT T2. Accordingly, if a voltage difference of the drive TFT T2 (i.e., a difference between the supply voltage applied from the supply voltage source VDD and the gate voltage) is greater than the threshold voltage Vth of the drive TFT T2, the drive TFT T2 is turned on.
Therefore, within display systems including a plurality of EL panels 20, the threshold voltages Vth of the drive TFTs T2 of the plurality of EL panels 20 must be equal to prevent the plurality of EL panels within the electro-luminescence display device from displaying pictures at different levels of brightness. Maintaining substantially identical threshold voltages Vth across a plurality of EL panels can be difficult because threshold voltages Vth of drive TFTs T2 typically vary with the manner in which the TFTs were fabricated. Accordingly, values of threshold voltages Vth of drive TFTs T2 can often vary from EL panel 20 to EL panel. If threshold voltages of drive TFT T2 in different EL panels 20 are different, the brightness at which pictures are displayed by EL panels within the electro-luminescence display device becomes undesirably non-uniform.
For example, a drive TFT T2 in a first EL panel of an electro-luminescence display device can have a threshold voltage Vth of 0.7V while a drive TFT T2 of a second EL panel of the electro-luminescence display device can have a threshold voltage Vth of 0.3V, wherein the supply voltage source VDD of the electro-luminescence display device applies a supply voltage of 10V. In the presence of an applied gate voltage of 9.5 V, the second EL panel may emit light because the voltage difference of the drive TFT T2 (i.e., 10V−9.5V=0.5V) is greater than the threshold voltage of the drive TFT T2 of the second EL panel (i.e., 0.3V). However, no pictures can be displayed by the first EL panel because the voltage difference of the drive TFT T2 (i.e., 10V−9.5V=0.5V) is lower than the threshold voltage of the drive TFT T2 of the first EL panel (i.e., 0.7V). Accordingly, the brightness with which pictures are displayed by the first and second EL panels within the electro-luminescence display device is undesirably non-uniform due to differences in threshold voltages Vth of the related art drive TFTs T2 within the first and second EL panels.